The present invention relates to a switching circuit, and more particularly, to a switching regulator having improved transducing efficiency.
Most electronic devices such as video tape recorders (VTR) and video cameras, whether they are portable or installed on vehicles on the like, use batteries as driving power. But the terminal voltage of batteries may vary greatly when a sudden change occurs in the load as a result of mode switching. To suppress these variations and stabilize the operation of the electronic devices, switching regulators are currently used.
FIG. 1 shows a conventional step-down switching regulator. A d.c. input Vi supplied across input terminals 2A and 2B has any ripple component removed in a capacitor 4 provided between the two input terminals. The input then is converted into an a.c. voltage by a switching transistor 6. The a.c. voltage is rectified in a d.c. reproducing circuit 8 composed of a diode 12, a choke coil 14 and a capacitor 16. Subsequently, the d.c. voltage is drawn from output terminals 10A and 10B as a stabilized d.c. voltage Vo.
The d.c. output Vo developed across the terminals 10A and 10B is supplied to the inverting input terminal of a comparator 18, and to detect any variation in the output, it is compared with a reference voltage provided at the noninverting input terminal from a power supply 20. The noninverting input of the comparator 18 is also provided with an a.c. signal positively fed back from the collector of the switching transistor 6 through a resistor 22. Therefore, in the switching regulator of FIG. 1, the comparator 18 operates both as an error amplifier and oscillator, and functions as a pulse width modulating circuit that generates pulses having an ON-OFF duty ratio adaptive to variations in the level of the output Vo. The output pulse from the comparator 18 is applied as a control pulse to the base of a transistor 30 that supplied a drive current for the switching transistor 6.
In the switching regulator of FIG. 1, the switching operation of the transistor 6 exhibits both rise and fall times as well as a certain delay time with respect to a pulsive input. These delay features in the pulsing operation cause a delay in the response time and also a power loss resulting from the current flowing in the delay period (the power loss is that current integrated by time). This power loss reduces the efficiency of d.c. to a.c. conversion accomplished by the switching regulator, and to avoid this problem, the delay in the switching time must be eliminated.
This demand for increasing the transducing efficiency of switching regulators has another justification. Recent electronic devices are designed to operate at a low voltage, and if the transducing efficiency of the switching regulator is low, the service life of the battery is shortened.
Furthermore, improving the input-output characteristics of switching regulators is essential in stablizing the electronic devices with which the regulators are used.